This invention relates to additive compositions, also known as admixtures, for incorporation in hydraulic cement mixes, for example, Portland type cement concretes and mortars and dry mixes for making such concretes and mortars.
Many admixtures which act as set retarders are employed primarily to achieve water reduction and improved compressive strength. As retarders, such admixtures slow the chemical process of hydration so that the concrete remains plastic and workable for a longer time than concrete without such a retarder. For example, a retarding admixture would be used to delay the set of the cement during difficult placements that require the concrete to be in a plastic condition for a longer than normal period of time to overcome undesired acceleration of set during hot weather.
Among the materials commonly used as set retarders, but which also are used for water reduction and improved compressive strength, are the lignosulfonates, such as calcium lignosulfonate; salts of hydroxycarboxylic acids; sugars such as glucose (dextrose), maltose and the like; and highly polymerized polysaccharides, such as dextrins. Additionally, starch, cellulose, phosphoric acid, and silicofluorides are used as retarding agents for retarding the setting time of cement mixes.
Retarding admixtures are frequently not used by themselves because either the dosage range is not practical and/or economical or, when there is the slightest variation in the amount, the mixing procedure or temperature, the admixture effects a greater degree of retardation of the rate of set or hardening, and sometimes of early strength, of the concrete or mortar than is desired. For example, when the lignosulfonates are employed in a practical and economical dosage range, they may effect a greater degree of retardation that is desired, or greater air entrainment than is desired, in addition to reducing the amount of water needed and increasing the compressive strength. To overcome these undesirable effects, frequently the retarder is employed with other admixtures such as calcium chloride or triethanolamine which are well known accelerators, to balance out the excess retardation. Further, the retarders may be used in combinations so that the smaller dosages of the individual ingredients will mean that the adverse effects will be lessened. For example in U.S. Pat. No. 3,188,221, silicofluorides were combined with an acid, such as phosphoric acid, boric acid, acetic acid, succinic acid or mixtures thereof. In U.S. Pat. No. 3,317,327, a silicofluoride was combined with an organic surface active agent.
Thus, a need exists for an admixture which will be useful by itself to retard the rate of hardening of cement and/or concrete mixes without significantly reducing the amount of water required or significantly increasing the amount of air entrained in the mix or resulting in the creation of other problems by its mere usage.